The Single Radial Immunodiffusion (SRID) technique was adapted for testing rabies vaccines for potency. The glycoproteins from the Pitman-Moore (PM) and Flury-LEP strain of virus were purified and a sheep antiserum was prepared against the PM glycoprotein. We received antisera against the Flury-LEP and ERA-strains from NIBSC (London). The glycoprotein content of each vaccine strain tested was found to vary when tested using the different antisera and when compared to the International Reference Preparation (strain PM). However, if a homologous vaccine strain was used as the reference, the relative potency values remained constant. These data indicate that, ideally, the standardization of different strain of rabies virus vaccine requires a homologous reference for each vaccine strain. Vaccine lots containing PM, SAD and Flury-LEP strains were tested by SRID and compared to the manufacturers NIH results. These data were analyzed using correlation and regression analyses. These results showed that only the PM vaccines had a correlative relationship between the SRID and NIH tests. The results also suggest that, for the PM strain, the SRID it is a less variable means of determining vaccine potency. Future studies will concentrate on standardizing SAD and Flury-LEP vaccines and to correlate SRID results with immunogenicity studies for these vaccines. We are now participating in a WHO collaborative study to determine whether a satisfactory correlation exists between SRID and NIH tests for vaccine produced by several manufacturers and whether the correlation coefficient becomes higher when NIH values are compared with the amount of glycoprotein bound to the rabies virion.